A typical process for the separation and recovery of olefins from pyrolysis furnaces operated with feedstocks heavier than ethane, is known as the front end depropanizer and front end acetylene hydrogenation scheme. A brief review of the typical front end depropanizer process is in order.
Starting with the separation section 2 after the water quench, as shown on the simplified process flow diagram of FIG. 1, there are three stages 4,6,8 of conventional compression to raise the pressure of the process gas from just above atmospheric to a pressure of about 15 bars (210 psia). Condensed liquids, i.e. hydrocarbons and water, are separated.
The gas is then treated in a conventional two or three stage caustic wash tower 10 as shown in FIG. 2 for the removal of carbon dioxide and hydrogen sulfide. The gas is cooled and mildly chilled before entering the dryers 12,14. Water is removed completely.
The gas is then further chilled in propylene refrigerant exchanger 16 seen in FIG. 3, and enters the high pressure depropanizer 18 which does not really operate at high pressure but is only called that because there is also a low pressure depropanizer 20. The high pressure depropanizer 18 typically operates at a pressure of 12 bars (170 psia), and the low pressure depropanizer 20 at a pressure of 8.5 bars (120 psia).
The overhead of the high pressure depropanizer 18 is usually compressed in compressor 22 to a pressure of 38 bars (550 psia) and is then sent to the acetylene hydrogenation system 24 which typically consists of two or three adiabatic reactors in series with inter-cooler for the removal of the heat of reaction. The reactor effluent is cooled in cooling water exchanger 26 and partially condensed in propylene refrigerant exchanger 28. A portion of the condensate is used as reflux via line 30 for the high pressure depropanizer 18. The rest is sent to the demethanizer stripper 32 (see FIG. 4) via line 34.
In the stripping section 36 of the high pressure depropanizer 18 only ethane and lighter components are removed, resulting in a fairly low bottoms temperature of 56.degree. C. (133.degree. F.). The bottoms product is sent via line 38 to the low pressure depropanizer 20 where it is separated into C.sub.3 's and C.sub.4+. The C.sub.3 is used as reflux in the high pressure depropanizer 18 via line 30, while the C.sub.4+ is sent to the debutanizer (not shown) via line 40. Due to the low operating pressure, the bottoms temperatures in the depropanizers 18,20 are quite low, namely 56.degree. C. (133.degree. F.) and 71.degree. C. (160.degree. F.). Therefore, there is no fouling in either tower 18,20 or their respective reboilers 42,44.
The acetylene hydrogenation unit 24 is highly efficient and selective. The acetylene removal easily results in acetylene concentrations of less than 1 ppm in the final ethylene product while the ethylene gain amounts to 50% or more of the acetylene. Due to the high hydrogen content of the feed gas, no carbonaceous material is deposited on the catalyst. The catalyst needs no regeneration and thus the reactors 24 need no spares. Green oil formation is miniscule.
In the acetylene hydrogenation reactor 24 about 80% of the methyl-acetylene and 20% of the propadiene are converted to propylene. If the olefins plant produces polymer grade propylene the remaining C.sub.3 H.sub.4 can be easily fractionated into the propane product; the high conversion of methyl-acetylene and propadiene in the acetylene hydrogenation reactors obviates the need for an additional separate C.sub.3 H.sub.4 hydrogenation system.
The operational stability of the acetylene hydrogenation reactor 24 is enhanced by its location in the gross overhead loop of the depropanizer 18 and in the minimum flow recycle circuit of the fourth stage of compression 22. These factors reduce the acetylene concentration in the inlet to the reactor 24 and stabilize the flow rate irrespective of the furnace throughput.
The vapor and liquid from the reflux accumulator 46 of the high pressure depropanizer 18 flow to the chilling and demethanization section 48 (see FIG. 4). The liquid plus the condensate formed at -37.degree. C. (-35.degree. F.) is sent via respective lines 34 and 50 to the demethanizer stripper 32. The overhead vapor from the demethanizer stripper 32 plus the liquids formed at lower temperatures are sent to the main demethanizer 52 via respective lines 54 and 56. The tower 52 is reboiled by reboiler 58 with condensing propylene refrigerant, and reflux is condensed in heat exchanger 60 with low temperature ethylene refrigerant.
The respective bottoms products 62,64 of the two demethanizers 32,52, after some heat exchange which is not shown, enter the prior art deethanizer 66. The tower 66 recovers approximately 40 percent of the ethylene contained in the two feeds as high purity product. Sixty percent of the ethylene and all the ethane leave the tower 66 as a side stream 68 and proceed to the low pressure ethylene fractionator 70. The deethanizer 66 is reboiled by reboiler 74 with quench water and reflux is condensed in exchanger 76 with -40.degree. propylene refrigerant. The bottoms product 72 of the deethanizer 66 is a stream containing propylene, propane and the remaining C.sub.3 H.sub.4. It flows to a conventional propylene fractionator (not shown). Because of the ethylene fractionation in its top section 78, the deethanizer 66 has fifty more trays than a conventional deethanizer (without the side draw) which produces a mixed ethylene and ethane overhead product in line 80.
The ethylene fractionator 70 is a relatively low pressure tower typically operating at 4 bars (60 psia) with approximately 100 trays. It uses an open heat pump. Ethylene refrigerant is condensed in the reboiler 82 and is then used as reflux via line 84. Effectively, the reboiler 82 also serves as the reflux condenser. There are no reflux pumps and there is no reflux drum.
Other references of interest are U.S. Pat. Nos. 5,709,780 and 5,755,933, both to Ognisty et al.